TheOne, thanks for the circuit... it's small/simple, and my switch-on signal is already active low, so I guess I can eliminate the NPN transistor. However, I'm interested in *how* this circuit was designed. Care to explain? So far any tutorials I've found on MOSFETS only discuss the design of these with holes and doping p-channel material, etc, etc. There is a big gap of practical design knowledge (that I'm missing) between that theory and a working schematic.
Oznog, I generally prefer TO-220 packages (easier to heatsink) so I started with that and found the FDP6030L. The datasheet says 13mOhm RdsOn at 10Vgs, which is close enough to the 12V that I should actually get. That translates to a nice low 13mV drop at a full 1A output. And also only 13mW of power consumption. Nice. Is that all there is to it?
I've dropped the requirement for the 3.3V switch. Instead, I need to switch 5V at up to 5A. If I use the same part, I'm looking at .5W used, which is considerable (in terms of heatsinking). Instead I (should) have an FDP6676 around here that has an RdsOn of .0075Ohms at 4.5Vgs. Calcs with this one give me .188 mW used at a full 5A, which is much nicer. Is that really all there is to it?
Yes, this is a one-off device. But I do also prefer to design for worst-case operation. I usually also add in an additional safety margin, depending on what is being designed and the consequences.
On a side note, if I wanted to make a MOSFET switch *really* turn on, can I just use a negative voltage that the gate to essentially raise the Vgs? Thanks.